1. Field of the Invention
The present invention relates to a laser light source device and an inspection device.
2. Description of Related Art
Generally speaking, there are two widely known methods for inspecting masks for defects, i.e., a comparison inspection method in which a mask pattern is compared with design data (which is commonly called “Die-to-database comparison method”) and a pattern comparison inspection method in which same parts of two chips are compared with each other (which is commonly called “Die-to-die comparison method”). In either case, a pattern that is detected (i.e., obtained) by magnifying and projecting a microscopic part in a mask pattern on an image sensor surface of a CCD camera, a TDI camera, or the like by using an objective lens and a projecting lens is compared with design data or another pattern. It should be noted that it is necessary to illuminate the observation area on the mask pattern surface, and various ultraviolet-range laser devices are used as the light source for the illumination (which are commonly called “mask inspection light sources”). An outline of this technical matter is explained in, for example, the below-shown Non-patent Literature 1.
In particular, as a high-sensitive mask inspection device using a short-wavelength laser, a pattern inspection device using an ultraviolet pulsed laser device having a wavelength of 193 nm as a mask inspection light source has been developed. This pattern inspection device is shown in, for example, the below-shown Non-patent Literature 2.
The resolution, which influences the repeatability of a mask pattern projected on the image sensor surface, is in proportion to the wavelength of the laser light for the illumination and in inverse proportion to the numerical aperture NA of the objective lens used in the projection optical system. Therefore, the resolution may be improved by using laser light having a shorter wavelength or an objective lens having a higher numerical aperture NA.
As a light source of light having a wavelength shorter than 193 nm, a fluorine molecule laser device, which performs a laser operation at a wavelength of about 157 nm, has been widely known. However, the repetition rate of the fluorine molecule laser device is low as described later. Therefore, it is conceivable to use the sixth harmonic of laser light that oscillates at a wavelength of 1064 nm, i.e., ultraviolet light having a wavelength of 177 nm.
In particular, it has been known that by using a nonlinear optical crystal such as a KBBF (KBe2BO3F2) crystal or an RBBF (RbBe2BO3F2) crystal, the sixth harmonic of a solid-state laser device having a wavelength of 1064 nm can be obtained (e.g., Non-patent Literature 10). Note that the KBBF crystal and the RBBF crystal are shown in the below-shown Non-patent Literature 3. Further, the below-shown Non-patent Literature 4 also mentions that laser light having a wavelength of 175 nm can be obtained by using the KBBF crystal or the RBBF crystal.
Meanwhile, for an inspection light source used in a pattern inspection device, a laser device capable of performing a continuous operation (called “CW operation”, CW stands for Continuous Wave) or a laser device capable of performing a high repetitive operation equal to or higher than 1 MHz is often used in order to obtain a uniform projection image and to increase the inspection speed.
In particular, when a TDI (Time Delay Integration) camera is used, the mask is scanned while projecting an image on the image sensor surface of the TDI camera. Note that the TDI camera is an image pickup device in which CCD elements perform a TDI operation. For example, a TDI camera is explained in the below-shown Non-patent Literature 5.
In a TDI camera, signals are integrated in one direction in the image sensor, which usually has a rectangular shape, and the signals are thereby averaged. As a result, even when the energy of pulsed light generated by the mask inspection light source fluctuates to some extent, the influence of the fluctuations on the obtained pattern signals is reduced. Therefore, it is preferable to use a light source that emits pulsed light at a repetition rate much higher than 100 to 1,000 Hz, which are typical framerates in TDI cameras. In particular, a laser device capable of operating a pulse operation at a high repetition rate equal to or higher than 1 MHz is suitable as the mask inspection light source. It should be noted that a CW operation laser device may be used. However, there is no CW laser device that can output sufficient power to carry out a mask inspection in the order of several hundred mw or higher at a wavelength shorter than 193 nm.
The reason why a laser device having a high repetition rate of 1 MHz of higher is suitable as a mask inspection light source is explained hereinafter. Assume an example case where the number of pixels of an image sensor is 1,000 pixels in the vertical direction and 2,000 pixels in the horizontal direction, and the image sensor is operated at a framerate of 1,000 Hz. In the case where the scanning direction in the mask is in parallel with the vertical direction of the image sensor, by using a light source capable of operating at a rate of 1,000×1,000 pulses per second (i.e., 1 MHz), the obtained values are averaged over the number of pulses equivalent to the number of pixels in the longitudinal direction. That is, optical energy quantities corresponding to 1,000 pulses are integrated at the same point. Therefore, even if the pulse energy fluctuates to some extent, its influence is substantially negligible.
It has been widely known that excimer laser devices and fluorine molecule laser devices perform high power operations in ultraviolet ranges and are used as lithography light sources. However, the repetition rate of the excimer laser devices and fluorine molecule laser devices is around 6000 Hz at the highest. Therefore, their repetition rate is too low to be used as a light source of a pattern inspection device, thus making them unsuitable for the pattern inspection device.
Meanwhile, though depending on the application, a pulsed laser device capable of operating at a very high repetition rate of 1 MHz or higher can be used as if it is a laser device capable of performing a continuous oscillating operation (CW). Therefore, in the sense that such laser devices are Quasi-CW laser devices, they are often called “QCW (Quasi-CW) laser devices”. Examples of the QCW laser devices include a solid-state laser device performing a mode-locked operation, and a laser device in which mode-locked operation solid-state laser light or laser light generated by a semiconductor laser device performing a pulse operation by performing a high-speed modulation is amplified by a fiber amplifier. For example, mode-locked laser devices of 76 to 100 MHz are commercially available from a number of laser device manufacturers. These QCW laser devices are suitable for inspection devices because they have a sufficiently high repetition frequency. A QCW laser device is shown in, for example, the below-shown Non-patent Literature 6.
Non-patent Literature 1: Sakuma Jun, DUV Laser Sources for Semiconductor Inspection Tools, Laser Engineering, Vol. 41, No. 9, pp. 697-707, 2013.
Non-patent Literature 2: Proceedings of SPIE Vol. 8701, p. 8701W, 2013.
Non-patent Literature 3: Colin McMillen, et al, “Hydrothermal Growth and Properties of KBe2BO3F2 (KBBF) and RbBe2BO3F2 (RBBF) Single Crystals, 2010 OSA Optics and Photonics Congress, NThC6, 2010
Non-patent Literature 4: J. Kilmer, et al., “Laser Sources for Raman Spectroscopy, “Proceedings of SPIE Vol. 8039, p. 803914, 2011
Non-patent Literature 5: Characteristics and Use of FFT-CCD Area Image Sensor, technical material, Hamamatsu Photonics K.K.
Non-patent Literature 6: technical material of Spectra-Physics, “Quasi-CW Solid-State Laser, “PHOTONICS SPECTRA, December 2002
Non-patent Literature 7: H. Friedman, Physics of the Upper Atmosphere, John Ashworth Ratcliffe, Academic Press, 1960
Non-patent Literature 8: Romain Royon, et al., “High power, continuous-wave ytterbium-doped fiber laser tunable from 976 to 1120 nm,” OPTICS EXPRESS 13818, 2013
Non-patent Literature 9: T. Matsui, et al., High resolution absorption cross-section measurements of the Schumann-Runge bands of O2 by VUV Fourier transform spectroscopy, Journal of Molecular Spectroscopy, Vol. 219, pp. 45-58, 2003
Non-patent Literature 10: Xin Zhanga, Lirong Wanga, b, Xiaoyang Wanga, Guiling Wanga, Corresponding author contact information, E-mail the corresponding author, Yong Zhua, Chuangtian Chena, “High-power sixth-harmonic generation of an Nd:YAG laser with KBe2BO3F2 prism-coupled devices,” Optics Communications, Volume 285, Issues 21-22, 1 Oct. 2012, Pages 4519-4522
However, the present inventors have found the following problem. When an ultraviolet light source having a wavelength of 177 nm is used as a mask inspection light source, the emitted ultraviolet light tends to be absorbed by oxygen contained in the air. Therefore, there is a concern that the laser light could be attenuated and thus necessary power could not be obtained. This is because, as shown in an absorption spectrum of oxygen shown in FIG. 8, the wavelength 177 nm is included in absorption bands called “Shumann-Runge Bands”. Note that the absorption spectrum of oxygen shown in FIG. 8 is shown in Non-patent Literature 7.
One of the techniques for preventing the attenuation of laser light is to fill the area through which the laser light passes with nitrogen. However, to fill the area with nitrogen, it is necessary to construct the inspection device with an air-tight structure. Further, it is necessary to evacuate the device by using a vacuum pump and then to fill the device with nitrogen. That is, it is necessary to construct the inspection device having a structure strong enough to withstand the pressure difference of one atmosphere (hereinafter called a “vacuum-tight structure”) so that the inspection device can be evacuated. This causes a problem that the device cost significantly increases.
Further, a large number of complicated optical components are used in the inspection device, and these optical components are fine-tuned by an engineer when the device is started up. For the fine-tuning, the engineer needs to open the cover of the device and look into the device. In this case, it is necessary to restore the pressure inside the vacuum chamber to the atmosphere for the fine-tuning, thus requiring a longer time for the fine-tuning.
The present invention has been made in view of these circumstances, and an object thereof is to provide a laser light source device capable of oscillating (i.e., generating) laser light that can efficiently propagate through the air, and an inspection device having a simple configuration.